hot water/shabbos

Question

you replied "same as a private house," I guess that means that it is prohibited, since it is not permitted in a private home. HOwever, I heard the following argument: the furnace in apartment buildings is so large that it is either 1) always pumping hot water or 2)when you turn on your hot water it does not cause the furnace to start generating more hot water (excuse my terminology, I am not a chemical engineer). In both of these scenarios it would seem that there is no prohibited melacha in turning in the hot water in an apartment building. In a house, however, because of the small size of the furnace, each time you turn on the hot water it causes the furnace to generate hot water. Have you heard this before? If not, does it make sense?